"Are the Planets Inhabited?" by E. Walter Maunder is a scientific publication written in the early 20th century. The book explores the possibility of life existing on other planets within our solar system, particularly focusing on the physical conditions necessary for habitability. The author presents a thorough examination of celestial bodies, employing a blend of observations and scientific reasoning to assess whether these worlds might support life akin to what we experience on Earth. The opening of the book establishes the framework for the inquiry into planetary habitability, addressing the historical context surrounding humanity's perception of the universe. Maunder elaborates on the evolution of thought regarding the nature of celestial bodies, asserting that for a planet to be deemed "inhabited," it must host intelligent, conscious life. He also emphasizes that the investigation must be based primarily on physical conditions and the presence of organic life-supporting elements, such as water. The initial chapters reflect on the definitions of life, the requirements for existence, and pose critical questions that will set the stage for evaluating planets like Mars and Venus in subsequent discussions. (This is an automatically generated summary.)